Which core material would make the strongest electromagnet?

The core material that would make the most powerful electromagnet is one that has high magnetic permeability and low coercivity.

Magnetic permeability is a measure of how easily a material can be magnetized. The higher the permeability, the more magnetic flux a material can carry. Coercivity is a measure of how difficult it is to demagnetize a material. The lower the coercivity, the easier it is to magnetize and demagnetize a material.

The best core materials for electromagnets are those made from soft iron. Soft iron has a high permeability and low coercivity, which makes it easy to magnetize and demagnetize and allows it to carry a large amount of magnetic flux.